Processes and compositions for the reduction of nitrogen oxides in a combustion effluent have been developed extensively over recent years. With the increased attention to the health risks and environmental damage caused by agents such as smog and acid rain, it is expected that NOx reduction research will continue to be pursued.
In an early application of the use of nitrogenous treatment agents to reduce nitrogen oxides, Lyon in U.S. Pat. No. 3,900,554, describes a process for reducing nitrogen monoxide (NO) from combustion effluents by introducing ammonia or certain "ammonia precursors" into the effluent at temperatures which range from 1300.degree. F. to 2000.degree. F. In U.S. Pat. No. 4,208,386, Arand, Muzio, and Sotter improve on the Lyon process by teaching the introduction of urea for NOx reduction in oxygen-rich effluents at temperatures in the range of 1600.degree. F. to 2000.degree. F., when urea is introduced into the effluent alone, and 1300.degree. F. to 1600.degree. F. when urea is introduced with an ancillary reducing material. Arand, with Muzio and Teixeira, also teach the introduction of urea into fuel-rich combustion effluents to reduce nitrogen oxides at temperatures in excess of about 1900.degree. F. in U.S. Pat. No. 4,325,924.
More recently, in a unique application of NOx reducing principles, Epperly, Peter-Hoblyn, Shulof, Jr., and Sullivan, in U.S. Pat. No. 4,777,024, teach a method for achieving substantial nitrogen oxides reductions while minimizing the production of so-called secondary pollutants, such as ammonia and carbon monoxide, through a multiple stage injection process. Moreover, Epperly, O'Leary, and Sullivan, in U.S. Pat. No. 4,780,289, have disclosed a complementary process for achieving significant, and potentially maximized, NOx reductions while minimizing the production of secondary pollutants. This process proceeds by utilizing the nitrogen oxides reduction versus effluent temperature curve of the treatment regimen being effected at each NOx reduction introduction in a combustion system.
In U.S. Pat. No. 4,861,567, Heap, Chen, McCarthy, and Pershing have disclosed a process which involves decomposing cyanuric acid in a fuel rich zone at 1000.degree. F. to form isocyanic acid and other products, which are then introduced into a combustion effluent for the reduction of nitrogen oxides and sulfur oxides (SO.sub.x). Furthermore, Azuhata, Kikuchi, Akimoto, Hishinuma, and Arikawa indicate in U.S. Pat. No. 4,119,702 that NO.sub.x reductions can be achieved at lower temperatures (i.e., 200.degree. C. to 800.degree. C.) by facilitating the decomposition of urea to NOx-reducing radicals by injecting an oxidizing agent with urea.
In addition, Hofmann, Sprague, and Sun have disclosed in U.S. Pat. No. 4,997,631 that the introduction of ammonium carbamate into an effluent can achieve substantial nitrogen oxides reductions while avoiding the presence of nitrous oxide.
Schell, in U.S. Pat. Nos. 4,087,513 and 4,168,299, discloses processes for the hydrolysis of urea to ammonia and carbon dioxide to eliminate urea from the waste water stream formed during urea production. These processes involve introducing the waste water stream into a carbon dioxide recovery system, optionally in the presence of vanadium pentoxide.
These patents, though, do not suggest the use of urea hydrolysis products for nitrogen oxides reduction, and especially not the use of a unique urea hydrolysate for NOx reduction.
Although as discussed above, U.S. Pat. No. 5,240,688 and International Publication No. WO 92/02450 discuss the use of the hydrolysis products of urea for nitrogen oxides reduction, both indicate the need for the application of pressure during hydrolysis. Neither one suggests that hydrolysis can be effected under low pressure, even within the effluent.
What is desired, therefore, is a system whereby nitrogen oxides reductions can be achieved using the hydrolysis products of urea, without the need for the application of pressure during hydrolysis. Also desired are a wider temperature window of NOx reduction, lower CO formation, N.sub.2 O generation and NH.sub.3 slip, and higher chemical utilization. This process should exhibit flexibility with reaction kinetics and residence time.